1. Field of the Invention
The present invention relates to a magnetic recording apparatus, and more particularly to a magnetic recording apparatus, such as a video tape recorder (VTR) of a helical scanning system, comprising a rotary erase head for performing smooth editing of tape such as perfect assembly recording and insert assembly recording.
2. Description of the Prior Art
In order to perform smooth editing of tape such as perfect assembly recording and insert assembly recording in a VTR of a helical scanning system, it is important that a rotary erase head scans a tape immediately before a recording head scans the tape so that erasing and recording can, be repeatedly, in that order, performed for each individual picture.
FIG. 1 is a perspective view showing one example of such a rotary erase head used in a 8 mm VTR utilizing a metal tape having high coercive force to obtain a high recording density, and FIG. 2 is a plan view showing diagrammatically a portion in the neighborhood of a gap of the rotary erase head. The rotary erase head as shown in FIGS. 1 and 2 is disclosed in, for example, an article, IEEE Transaction on Magnetics MAG-20, 1984, pp. 869-871.
The structure of the rotary erase head shown in FIGS. 1 and 2 is described below. In FIG. 1, halves 1 and 2 of a Mn-Zn ferrite core having a saturation magnetic flux density of approximately 5000G are provided opposed to each other, the side of the ferrite core half 2 corresponding to the forward side with respect to the travelling direction of the head. A Sendust film 3 having a saturation magnetic flux density of approximately 10000G is formed by a sputtering process to a thickness of approximately 3 .mu.m on the surface of the ferrite core half 2 opposed to the ferrite core half 1. A gap 4 having a gap length G of approximately 3 .mu.m is formed between the Sendust film 3 and the surface of the ferrite core half 1. A film 5 of non-magnetic materials is formed in the gap 4. In addition, a glass layer 6 is formed on both sides of the gap 4 so that it may extend outwardly. A coil (not shown) is wound through a hole 7 and notches 8 and 9. A rotary erase head 10 is thus completed.
FIGS. 3 and 4 are a plan view and a side elevational view, respectively, illustrating the state in which the rotary erase head shown in FIG. 1 is mounted to a rotary cylinder in a conventional VTR. As shown in FIG. 3, recording heads A and B each having different azimuth are attached to the position apart from each other by 180.degree. in the circumferential direction around the rotary cylinder 13. In addition, the rotary erase head 10 is attached to the position apart from the recording head A by 90.degree., in the middle position between the recording heads A and B, so that it may scan the forward portion of the tape with respect to the recording head A. A magnetic tape 14 is spirally wound around a head cylinder system comprising the cylinder and heads. As shown in FIG. 4, the dimension in the track-width direction TW (or the direction of the thickness of the rotary cylinder 13) of the rotary erase head 10 is twice the dimension in the track-width direction of each of the recording heads A and B. Furthermore, the rotary erase head 10 is displaced, by a distance Z corresponding to a half of a track pitch, upward from the recording heads A and B with respect to the axial direction of the rotary cylinder 13.
FIG. 5 is a diagram showing diagrammatically the principle of recording and erasing by the conventional VTR shown in FIGS. 1 to 4. Referring now to FIGS. 1 to 5, the recording and erasing operation of the head cylinder system of the conventional VTR is described.
First, as shown in FIG. 4, it is assumed that the magnetic tape 14 spirally wound around the rotary cylinder travels diagonally, from top to bottom, with respect to the rotating direction 12 of a head. In this case, the recording heads A and B are attached to the rotary cylinder 13 so that the respective ends are located forward with respect to the tape travelling direction 11, that is, located on the lower side as shown in FIG. 4, of the respective both ends in the track-width direction of both the recording heads A and B may be in the same level with respect to the axial direction of the rotary cylinder 13. Thus, a recording track 15A by the head A and a recording track 15B by the head B are formed alternately on the magnetic tape 4 with a track pitch TP (the center-to-center distance between adjacent tracks) determined based on the tape travelling speed, as shown in FIG. 5. As described above referring to FIG. 4, each head is arranged so that the end located forward with respect to the tape travelling direction, that is, located on the lower side in FIG. 4, of the both ends in the track-width direction of the gap 4 in the rotary erase head 10 (the surface abutting to the magnetic tape) is placed backward with respect to the tape travelling direction (on the upper side in FIG. 4), by the distance Z which is a half of a track pitch with respect to the axial direction of the rotary cylinder 13, as compared with the ends located forward with respect to the tape travelling direction (located on the lower side in FIG. 4), of the respective both ends in the track-width direction of the heads A and B. Therefore, when the rotary erase head 10 is operated, erasing and recording are repeatedly performed for each one picture. For example, erasing two tracks by the rotary erase head 10 and subsequently recording by the head A (15a in FIG. 5) and recording by the head B (not shown) on the erased tracks are repeatedly performed, as shown in FIG. 5.
However, since the gap length G of the above described rotary erase head 10 is approximately 3 .mu.m, which is approximately ten times the gap length of the recording head, and the respective both ends in the track-width direction of the Sendust film 3 and the ferrite core half 1 on the respective sides of the gap 4 are not necessarily coincident with each other, disturbance of erasing magnetic flux produced from the gap 4 (the fringing effect in the neighborhood of both ends in the track-width direction of the gap 4) can not be ignored. More specifically, as shown in FIG. 2, if the width T, of the portion in which magnetic materials on both sides with respect to the gap 4 are completely opposed to each other, is considered as the effective track width of the rotary erase head 10, there occurs a problem in which, with the arrangement of the heads shown in FIG. 4, a part of the recorded pattern 15B (in FIG. 5), formed adjacent to the erase head by the head B scanning the tape immediately before erasing by the rotary erase head 10, is erased by fringing magnetic flux from the rotary erase head 10. That is also obvious from the following Table 1. The Table 1 represents measured data showing a phenomenon in which almost the same recording and reproducing outputs can be obtained from the recording track recorded by the head A scanning the tape immediately after scanning by the rotary erase head 10, irrespective of whether or not the rotary erase head 10 is operated, whereas in the recording track recorded by the head B scanning the tape immediately before scanning by the rotary erase head 10, recording and reproducing outputs are largely reduced in the case where the rotary erase head 10 is operated, as compared with the case where it is not operated.
TABLE 1 ______________________________________ recording and reproducing recording and reproducing outputs of 1 MHz (record- outputs of 6.5 MHz (re- ing current 50 mAp-p) cording current 25 mAp-p) head A head B head A head B ______________________________________ with 257 mVp-p 195 mVp-p 92 mVp-p 76 mVp-p erasing without 255 mVp-p 253 mVp-p 95 mVp-p 98 mVp-p erasing ______________________________________
In addition, the recording pattern on the tape when the rotary erase head 10 is operated was observed by applying ferromagnetic colloid thereto and it could be found that a part of the recording track by the recording head B had been erased. Therefore, it can not be expected for the head cylinder system of the conventional VTR as described above to perform not only clear perfect assembly recording and insert assembly recording but also normal recording and reproducing.
On the other hand, FIG. 6 is a plan view showing diagrammatically a rotary erase head having a structure similar to that of the conventional rotary erase head shown in FIGS. 1 and 2, particularly showing a structure in the neighborhood of the surface abutting to the magnetic tape. The rotary erase head shown in FIG. 6 is applied to the 8 mm VTR similarly to the rotary erase head shown in FIGS. 1 and 2 and is disclosed in Japanese Patent Laying-Open Gazette No. 89807/1985, for example. In FIG. 6, a pair of ferrite core halves 1 and 2 opposed to each other to form the abutting surface shown in FIG. 6 are the same as those shown in FIGS. 1 and 2. The Sendust film 3 is previously formed on the ferrite core half 2. The Sendust film 3 may be replaced with a metal magnetic material film such as an amorphous and a permalloy. A gap between the ferrite core half 1 and the Sendust film 3 forms an operating gap 16. A groove 17 for regulating a track width and expanding outwardly is formed in the both side portions of the operating gap 16. The gap length G of the operating gap 16 is selected to be approximately constant, and the track width Ts on the side of the Sendust film 3 is selected to be slightly larger than the track width T.sub.F of the ferrite core half 1. The operating gap 16 and the groove 17 are filled with a glass material 18. The glass 18 in the groove 17 encapsules the area abutting the tape and provides protection for the operating gap 16.
FIG. 7 is a graph showing the erasing property of the rotary erase head shown in FIG. 6. In FIG. 7, the abscissa represents the amplitude of erasing current of 6 MHz flowing into the above described rotary erase head, the ordinate represents the erasing rate, (Y) represents the erasing property of a luminance signal, (C) represents the erasing property of a chrominance signal and (P) represents the erasing property of a pilot signal for tracking, respectively. See, for example, U.S. Pat. No. 4,297,733 for the pilot signal for tracking. For example, a rotary erase head used in a VTR must be able to erase one arbitrary frame so as to obtain clear perfect assembly recording or insert assembly recording as described above. To this end, a sufficient erasing rate and a stable erased width are required. In order to obtain such a sufficient erasing rate, a current of more than 150 mA, that is, current approximately ten times as large as normal recording current must be required, as obvious from FIG. 7.
However, as shown in FIG. 6, a rotary erase head adapted such that the gap length G of the operating gap 16 is approximately constant and the track width Ts on the side of the Sendust film 3 is slightly larger than the track width T.sub.F on the side of the ferrite core half 1 encountered a problem that if erasing current approximately ten times as large as normal recording current flows as described above, the leakage magnetic field becomes larger and the actual erased width becomes larger than the apparent track width T.sub.F.
In addition, there is also a problem that the actual erased width depends on the track width T.sub.S on the side of the Sendust film 3 rather than the track width T.sub.F on the side of the ferrite core half 1 and tends to change according to the erasing current value. More specifically, FIG. 8 is a graph showing a relation between erasing current (abscissa) of the rotary erase head shown in FIG. 6 and the expansion of the erased width (ordinate). As shown by a solid line in FIG. 8, the expansion of the actual erased width is approximately constant in the range of more than 400 mA of erasing current and it linearly changes in the range of 150 to 300 mA thereof. Therefore, in case of erasing current ten times as large as recording current, that is a current of 50 to 300 mA, the expansion of the actual erased width changes largely in response to a slight change in the erasing current value. Furthermore, since in many cases, an erasing current circuit for such a rotary erase head uses a resonance circuit utilizing a coil of the erase head, there has been a problem that erasing current changes according to a slight change in inductance of a magnetic head and Q of the resonance circuit, so that the erased width changes. Therefore, if and when the rotary erase head having a structure of an unstable, actual erased width is attached to the rotary cylinder 13 as shown in FIGS. 3 and 4 and employed, the fringing effect adversely affects the adjacent recording patterns formed immediately before erasing by the rotary erase head, which becomes significant. To solve these problems, it is desirable to require the strict standard of inductance of the above described magnetic head and Q of the resonance circuit and reduce the variation thereof. However, it is difficult in respect of the yield and the productivity. If erasing current is rendered large, the erased width becomes stable as shown in FIG. 8. However, it is not preferable in respect of the reliability of an electric circuit and the consumed power.